UV (ultraviolet) induced fluorescence continues to be one of the most promising techniques for the real time detection of biological agents and other particles. A number of detectors developed around this technique have shown that UV induced fluorescence provides a means to rapidly and accurately detect biological agents at very credible limits of detection. Among these devices are the Biological Agent Warning Sensor (BAWS) developed by MIT Lincoln Laboratory, along with several others described by U.S. Pat. Nos. 5,701,012; 5,895,922 and 6,831,279. Additional devices are described in U.S. Pat. Nos. 5,999,250; 6,885,440; 6,967,338; 7,375,348; 7,567,391 and 7,852,469.
These detectors work under the principle that aerosolized biological agents will fluoresce and scatter light when excited with UV light. The UV light source can be a laser, LED with optics, or any other emission source that can produce a beam of light which can then be pointed towards aerosol particles. The excitation wavelengths are typically in the 405 to 266-nm wavelength range but are not limited to that range.
Aerosol particles will fluoresce when hit with an excitation beam due to biochemicals, specifically fluorophores, contained within the biological agents. The fluorescence is at a wavelength longer than the excitation wavelength. Tryptophan, for example, a common component of biological materials, has a peak fluorescence in the 350-nm range when excited with 266-nm light. The scattering wavelength is the same as the excitation wavelength, in this case 266-nm. Both the fluorescent and scattering light are detected using optical detectors such as a photomultiplier.
The relative amount of fluorescent and scattering light emitted from the biological aerosols can be characterized by the scattering and fluorescent cross sections ofthese materials. In general, the scattering cross sections are several orders of magnitude greater than the fluorescent cross sections.
The basic principles of detector operation are described in connection with FIG. 1A which shows a top view of a particle detector and FIG. 1B which shows a side view of a detector. Referring to FIG. 1A, a source 10 of UV light generates a UV beam 12 which intersects at 14 with a stream 16 of particles being pulled into the detector. Intersection 14 occurs within a mirrored chamber 18. A beam dump 20 captures excess light from beam 12 so that it is absorbed and removed. As shown in FIG. 1B, the intersection 14 between UV beam 12 and particle stream 16 generates scattered and fluorescent light beam 22. Beam 22 enters the region of the particle detector that transmits the scattering and fluorescence signals from the particle to the photomultiplier optical detectors. This includes beam splitter 24 which divides the beam into scattered component 26 and fluorescent component 28. Filter 30 removes extraneous scattered light from fluorescent component 28. Photomultiplier 32 records the intensity of scattering component 26 while photomultiplier 34 records the intensity of fluorescent component 28. While this device is described using photomultipliers; other optical detectors such as avalanche photodiodes (APDs) may be used. FIGS. 1A and 1B depict a general overview of particle detector operation; many other components could be used as needed for a specific situation.
A great deal of design effort has been employed to attenuate the amount of stray excitation light that can inadvertently make it to photomultipliers 32 and 34 via the optical train. The optical train of a particle detector includes all internal surfaces that have a direct or indirect path between light source 10 and photomultipliers, or optical detectors 32 and 34.
Stray light within the optical train can have two effects. First it can be falsely recorded as a scattering signal given that stray light and excitation light are both at the same wavelength. This signal will then appear as a scattering signal in scattering photomultiplier 32. Second, and more importantly, the scattered light can cause other objects and materials in the overall optical train to fluoresce. This signal will then appear as a fluorescent signal in the fluorescent photomultiplier 34.
Two main ways in which stray light can be attenuated are by using spatial filters and by eliminating reflective surfaces. In the prior art, reflective surfaces have been reduced by applying absorptive coatings or by increasing the volume of the optical train and placement of its components to the point that any reflected light would have an unlikely probability of reentering the pathway leading to the optical detector.
Those knowledgeable in the art of reducing stray light within optically based sensors and detectors understand that this represents a significant design challenge and, in general, the best solution is often a compromise as opposed to a perfect solution. This is caused by the fact that the sensor and detector design options are usually bounded by size, weight, and cost constraints. In principle, the entire optical train could be produced from any materials and coatings that result in an end product that addresses the need to attenuate the stray light and optical train fluorescence. In practice, however, this is often limited by cost and manufacturing constraints.
Prior art particle detectors working in the deep UV region and designed to detect biological materials typically avoid the use of uncoated plastics within the detector's optical train. This is especially true in the 266-nm region where plastics are known to fluorescence. This auto-florescence is significant and may easily mask the fluorescence from biological materials at the same wavelength. Plastics may also have reflective properties that causes undesired scattering of light throughout the optical train. In this case, they can appear as reflective surfaces and effectively act as shiny surfaces.
However, it is recognized that the plastics offer several benefits over non-plastic approaches. Compared to alternative approaches such as machined metals, plastics provide lower manufacturing and materials costs. They are also lighter, more rugged, and easier to assemble. However, the auto-florescence properties of available plastics have limited the ability to exploit these advantages.
While plastics offer some significant manufacturing, size, weight and cost advantages for an optical train, this choice of material has not been pursued in an integrated detector due to their reflective and fluorescent characteristics. Many plastics are inherently smooth and very reflective. There are, however, available techniques such as sanding to reduce this feature. A bigger impediment to the use of plastics in detectors is the fact that they inherently fluoresce when excited with UV light. Optically based detectors have been designed around the fluorescent quality of plastics. For example, Deep-UV LED and Laser Induced Fluorescence Detection and Monitoring of Trace Organics in Potable Liquids. WO 201204052 A2, teaches using detectors similar to those described in the patents listed above to detect a few parts per trillion of plastic resins in bottled drinking water and river plumes.
These and similar studies have generated a position within the detector development community that plastics cannot be used within UV based detectors, especially in regions where they could interact with the excitation light. The only noted exceptions were in applications where special plastics were used as a non-moving, solid support. The plastics were coated with immobilized binding ligands or similar materials. These coatings are known to produce an optical response when interrogated with UV light. In these cases, the plastic was not used or applied to the optical train in either the generation of the excitation source or the collection of the fluorescent and scattered light. The use was limited to an interrogated surface. For example. Ha Kim, et al. “Reusable low fluorescent plastic biochip: WO 2000055627 A1, teaches a non-auto-fluorescent solid support that is an alternative to glass that is suitable for the construction of biochips that can be employed in high-sensitivity, fluorescence detection and other methodologies. It identifies a UVT (Ultra-Violet Transmitting) Acrylic Ultraviolet Transmitting plastic, Glasfex® (now made by Spartech Polycast®) but does not teach if this material would function in the deep UV (<266-nm or lower) range necessary for the detection of biological constituents such as tryptophan. Data collected on similar acrylics in the deep UV would suggest that these materials would have unusable fluorescence levels at this and lower wavelengths. Regardless, even if it had been shown that UVT Acrylic Ultraviolet Transmitting plastic would function in the deep UV, Kim does not teach or suggest that such or similar material can be applied to the design and construction of the detector's optical train.
In principle, the adverse fluorescent properties of plastics could be overcome by applying a protective coating on the plastic thus shielding it from any light. The common practice of applying a metal coating would not, in itself, suffice given that this would introduce a higher level of reflected light into the optical train and adverse increase scattering signals. In addition, a coating with the proper reflective and fluorescent properties to use with a practical plastic optical train is not known, and it would certainly add cost and complexity to the design and manufacturing of plastic detector.
Thus, a need exists for an optical train in a particle detector that benefits from the reduced cost, lighter weight and ruggedness of plastics but does not have the disadvantages of reflectivity and fluorescing in deep UV wavelengths.